High output, quick response, electrical radiant heaters having heating elements in the form of thin corrugated metal foil ribbons are well known. Such metal foil ribbon heaters have a low thermal inertia and are particularly suited for employment in situations requiring high temperature, rapid heating of objects with quick cooling and low retention of heat in the heater unit when it is cut off, as when a moving line of objects are stopped temporarily in front of the heater and overheating is to be avoided. The metal foil ribbon heating elements generally are comprised of very thin corrugated metal strips, such as "Inconel" metal, and are mounted on a thermally insulating backing material, such as a ceramic block, with the exposed face of the ribbon directed toward the object or objects to be heated. Such heater units are suitably connected to a power source and generally are supported above or alongside a path of moving material, such as a moving sheet, web, or fabric, to heat the same.
One such radiant heater construction is described in U.S. Pat. No. 3,525,850, the disclosure of which is incorporated herein by reference. As described in the patent, an electrical resistance, corrugated metallic foil ribbon adapted to be heated by electricity to a temperature in the range of about 1200.degree. to 1800.degree. F. is backed by a thermally insulating backing, preferably a ceramic material, having a thermal conductivity at 1500.degree. F. in the range of about 0.07-0.15 B.t.u./hr.-ft.-.degree.F. The backing may be made of an assemblage of blocks to minimize warping or fracturing at high temperatures of operation of the heater.
Difficulties have been experienced in maintaining the structural integrity of the thin foil ribbon heaters due to several factors. Longitudinal elongation occurs in the thin metal ribbons during heating, causing the foil ribbons to droop and sag away from the insulating backing at high temperatures, resulting in uneven heating of the objects being heated. In addition, the thin ribbon elements frequently burn out when hot spots occur in the ribbons. U.S. Pat. No. 3,525,850 attemps to overcome the problems by employing fastening means composed of heat conductive wires, nails or pins spaced along the length of the ribbons to hold the ribbons to the thermally insulating backing, with the metal fastening elements extending through the insulating back to the rear side thereof to convey excess heat away from the foil ribbon at their points of contact with the fastners.
The use of wires, pins, and clamps along the length of the ribbons restricts free longitudinal expansion and contraction of the same. Such pins, clamps and wires also wear the foil ribbons at their points of contact, cutting into the thin edges of the metal foil and consequently decreasing the life of the heating element. Because the metal fasteners as described in U.S. Pat. No. 3,525,850 extend through to the rear side of the insulating material to dissipate heat, there is a heat loss and consequently loss in the effeciency of the heater unit. Further, exposure of electrically conductive metal wires, clamps, and pins on the back of the thermally insulating backing material creates electrical shock and fire hazard if the fasteners are contacted by foreign objects and materials, such as lint, dust or debris which may fall into or occur around the heating unit.